Pharmacology
General Pharmacology
General pharmacology for MBBS and NEET-PG: bioavailability, first-pass, zero and first-order kinetics, enzyme inducers and inhibitors, and therapeutic index, mapped to NMC codes.
MedNext Academy | 3 min read
General Pharmacology
General pharmacology for MBBS and NEET-PG: bioavailability, first-pass, zero and first-order kinetics, enzyme inducers and inhibitors, and therapeutic index, mapped to NMC codes.
General pharmacology sets the rules that every later drug chapter follows. It covers pharmacokinetics, how the body handles a drug through absorption, distribution, metabolism and excretion, and pharmacodynamics, how the drug acts on the body through receptors, agonism and antagonism.
High-yield: General Pharmacology
- Bioavailability is the fraction of a dose that reaches the systemic circulation; it is 100% for intravenous drugs by definition.
- First-pass metabolism in the gut wall and liver lowers the oral bioavailability of propranolol, morphine, lignocaine and glyceryl trinitrate.
- First-order kinetics eliminates a constant fraction per unit time; most drugs follow it at therapeutic doses.
- Zero-order kinetics eliminates a constant amount per unit time and applies to ethanol, phenytoin, aspirin at high dose and theophylline.
- Phenytoin shifts from first-order to zero-order kinetics within its therapeutic range, a classic exam point.
- Volume of distribution links dose to plasma concentration; a high value means extensive tissue binding and poor dialysability.
- Steady state is reached in about 4 to 5 half-lives, whatever the dosing interval.
- A loading dose depends on the volume of distribution; the maintenance dose depends on clearance.
- Phase I reactions are oxidation, reduction and hydrolysis, mainly by cytochrome P450; phase II reactions are conjugation.
- Enzyme inducers include rifampicin, phenytoin, carbamazepine, barbiturates and chronic alcohol; they lower drug levels.
- Enzyme inhibitors include ketoconazole, erythromycin, cimetidine, valproate and grapefruit juice; they raise drug levels.
- A competitive antagonist shifts the dose-response curve to the right and is surmountable; a non-competitive antagonist lowers the maximal response.
- The therapeutic index is the ratio of toxic to effective dose; warfarin, digoxin, lithium and phenytoin have a narrow index and need monitoring.
- Drugs with a high volume of distribution, such as digoxin, are not effectively removed by haemodialysis.
Enzyme inducers and inhibitors
- **Inducers (lower drug levels):** Rifampicin, phenytoin, carbamazepine, phenobarbitone, chronic alcohol, griseofulvin.
- **Inhibitors (raise drug levels):** Ketoconazole, erythromycin, cimetidine, ciprofloxacin, valproate, grapefruit juice, acute alcohol.
- **Zero-order drugs:** Ethanol, phenytoin, aspirin at high dose, theophylline.
- **Narrow therapeutic index:** Warfarin, digoxin, lithium, phenytoin, theophylline. Monitor levels.
NMC competencies in this chapter
- **PH1.1:** Pharmacokinetics: absorption, distribution, metabolism and excretion
- **PH1.4:** Bioavailability, first-pass metabolism and volume of distribution
- **PH1.7:** Drug metabolism, enzyme induction and inhibition
- **PH1.9:** Pharmacodynamics: agonists, antagonists and the dose-response curve
- **PH1.11:** Therapeutic drug monitoring and drugs with a narrow therapeutic index
Frequently Asked Questions
What is first-pass metabolism?
It is the metabolism of an oral drug in the gut wall and liver before it reaches the systemic circulation, which lowers oral bioavailability for drugs like propranolol and morphine.
Which drugs follow zero-order kinetics?
Ethanol, phenytoin, aspirin at high dose and theophylline. A constant amount is eliminated per unit time, so small dose increases can cause large rises in level.
How is a loading dose different from a maintenance dose?
A loading dose is set by the volume of distribution to fill the tissues quickly, while the maintenance dose is set by clearance to match elimination.
Why monitor narrow therapeutic index drugs?
Because the toxic and effective doses are close, small changes in level can cause toxicity, so warfarin, digoxin, lithium and phenytoin are monitored.
Continue reading
MBBSAll Pharmacology chapters
Continue through the Pharmacology chapter map.
Continue studying Pharmacology
Explore clinician-written learning resources, structured revision and practice across the MedNext platform.
Open in the MedNext appSee plans